Решение примеров:

a)

$$\frac{2}{5} - \frac{1}{4} + \frac{9}{20} = \frac{2*4}{5*4} - \frac{1*5}{4*5} + \frac{9}{20} = \frac{8}{20} - \frac{5}{20} + \frac{9}{20} = \frac{8 - 5 + 9}{20} = \frac{12}{20} = \frac{3}{5}$$

б)

$$\frac{7}{30} + (\frac{3}{5} - \frac{1}{6}) = \frac{7}{30} + (\frac{3*6}{5*6} - \frac{1*5}{6*5}) = \frac{7}{30} + (\frac{18}{30} - \frac{5}{30}) = \frac{7}{30} + \frac{13}{30} = \frac{7 + 13}{30} = \frac{20}{30} = \frac{2}{3}$$

в)

$$\frac{7}{8} - (\frac{1}{9} + \frac{2}{3}) = \frac{7}{8} - (\frac{1}{9} + \frac{2*3}{3*3}) = \frac{7}{8} - (\frac{1}{9} + \frac{6}{9}) = \frac{7}{8} - \frac{7}{9} = \frac{7*9}{8*9} - \frac{7*8}{9*8} = \frac{63}{72} - \frac{56}{72} = \frac{63 - 56}{72} = \frac{7}{72}$$

г)

$$(\frac{5}{14} + \frac{9}{10}) - \frac{5}{7} = (\frac{5*5}{14*5} + \frac{9*7}{10*7}) - \frac{5}{7} = (\frac{25}{70} + \frac{63}{70}) - \frac{5}{7} = \frac{88}{70} - \frac{5}{7} = \frac{88}{70} - \frac{5*10}{7*10} = \frac{88}{70} - \frac{50}{70} = \frac{88 - 50}{70} = \frac{38}{70} = \frac{19}{35}$$